Certain applications of lift trucks require an upright construction that is capable of providing both positive and negative lift from a ground or support level position. For example, such a lift truck is particularly useful for handling boats in and around marinas. The market for such a lift truck has significantly increased in recent years with ever more and more people owning and operating pleasure boats.
In the marina setting, lift trucks may be utilized to both lower boats into and raise boat out of the water from an elevated dock or the like. Similarly, such lift trucks may be utilized to raise boats for positioning well above the ground in an overhead storage rack.
Heretofore, lift truck designs have been developed for this purpose. One such representative design is disclosed in U.S. Pat. No. 3,841,442 to Erickson et al assigned to the Assignee of the present invention. The lift truck disclosed in the Erickson et al patent includes outer, intermediate and inner, telescoping mast sections with a load carriage elevatable on the inner mast section. The lift truck also includes a pair of actuator cylinders and cooperating chains. These cylinders and chains are connected to the mast sections so that one cylinder and chain set is adapted to elevate the load carriage and the inner mast section above ground level. The other cylinder and chain set is adapted to lower below ground level the load carriage and inner and intermediate mast sections together as a unit in the outer mast section.
While the lift truck disclosed in the Erickson et al patent allows positive and negative lift capabilities for effective utilization in a marina setting, it suffers from a number of distinct shortcomings shared with other state of the art lift truck designs. More specifically, the tandem lift cylinder and chain sets required for positive and negative lift are mounted between the uprights of the masts significantly limiting and in many cases substantially blocking forward visibility. In effect, the reduced forward visibility substantially limits the ability of the lift truck operator to properly orient the forks when picking a boat up or placing a boat in a storage rack berth. Accordingly, a significant potential exists that a boat may be damaged while being picked-up, carried and/or placed with the lift truck.
Other shortcomings of state of the art lift trucks include a relatively limited lifting capacity. This directly results from (1) the provision of separate actuator cylinder and chain sets for achieving positive and negative lift configurations; (2) the utilization of heavy gauge steel rails necessary to increase rigidity and (3) the forward location of the mast sections relative to the center axis of the lift truck. The weight of the cylinders and heavy gauge rails as accentuated by the forward placement requires the provision of a heavier, bulkier counterweight. The truck must also be made longer overall to provide a greater moment arm for the counterweight to be effective. Further a more powerful operating engine is also required to obtain the desired performance. Each of these requirements add substantially to the cost of lift trucks.
Additionally, state-of-the-art marina lift trucks commonly utilize complicated fork structures and controls. Unfortunately, the fork structures typically require maintenance at relatively short intervals to insure reliable operation. Such maintenance is particularly required at ocean marinas due to the corrosive properties of saltwater environments. Additionally, the complicated controls require the individual to receive extensive training before the lift truck can be effectively operated. Even when fully familiar with the operation of the controls, the manipulation of multiple levers as now required on state of the art lift trucks requires additional time thereby reducing the productivity of even a skillful operator.
Another problem typical of prior art lift truck designs relates to the need for an improved fork. Forks presently in use are typically constructed of steel for strength and include a protective cover on the upper surface to cushion and protect a boat hull from direct contact with the steel fork. It has been found, however, that such covers when pinched between the boat hull and the steel fork wear quickly and must be replaced after only a relatively short service life. Additionally, as the covers become worn they have a tendency to retain more and more water when manipulated to lift a boat from the water. Subsequently, when the boat is then positioned in an upper berth of a rack, the water retained in the covers drips down onto underlying boats. This water often includes contaminants such as rust from the forks and grease or oil from the dock side water. These contaminants may stain the finish and/or furnishings of underlying boats to the dissatisfaction of the boat owners. As a result, customer relations of the marina operator may be adversely affected.
From reviewing the above it is clear that a need exists for an improved lift truck providing positive and negative lift capabilities that is particularly adapted for operation in both coastal and inland marinas.